First performed 1 July 1977, as part of the Queen's Silver Jubilee, in a production directed by Trevor Nunn, with Ian McKellen as Alexander, John Wood as Ivanov, and Patrick Stewart as the Doctor, with Andr&eacute Previn conducting the London Symphony Orchestra

Every Good Boy Deserves Favour is an unusual collaborative piece, written by Stoppard at the behest of André Previn who then wrote the music to go along with it.
The play was written for a full orchestra, rather limiting the possibility for revivals, but Previn rescored it for a chamber orchestra and so it did enjoy some success after what had been planned simply as a one-off performance.
Every Good Boy Deserves Favour is a political play, set in the Soviet Union of that day (1977).
It is largely set in a mental institution, where two characters -- both named Alexander Ivanov -- share a cell.
One is called Alexander, the other Ivanov, but the names obviously cause some confusion.
Alexander is a dissident, institutionalized for his unacceptable views.
Ivanov is a genuine loon.
Ivanov imagines that he is playing the triangle in an orchestra -- an orchestra that does, in fact, exist in the staging of the play, if not in the reality it depicts.
The play is much the usual thing when plays are set in mental institutions: who is sane ?
what is sane, in this world ?
etc.
Of course, here there is an added political dimension, with the government placing those who oppose its views in such institutions.
It makes for quite a vicious circle:

DOCTOR: (...) For example, you are here because you have delusions that sane people are put in mental hospitals.

ALEX: But I am in a mental hospital.

DOCTOR: That's what I said. If you're not prepared to discuss your case rationally, we're going to go round in circles.

Alexander's son, Sacha, also gets into trouble at school ("Detention is becoming a family tradition", his teacher notes).
He tries to help his father, but his principled father won't admit to the errors of his ways.
Both sides -- state and individual -- stand by their principles, leading to something of an impasse.
One last predictable confusion of identities -- Alexander and Ivanov are once again mistaken -- leads to both patients being released at the end.
But the system prevails, even in failure.
(In Stoppard's Theatre (see our review) John Fleming notes that the final scene was restaged in later productions of the play, to make it absolutely clear that the Colonel that frees the two men is, in fact, completely aware that he is asking the wrong questions, allowing him to free Alexander without admitting any wrongdoing -- or acknowledging that the system is an unjust one.)

The play is cleverly conceived, with the usual bright and sharp Stoppard dialogue.
There's farce, there's tragedy -- and there's that orchestra (which is presumably better appreciated on the stage than on the page).
The outrageous Soviet policy of hiding away dissidents and silencing critics in this manner is fortunately largely a thing of the past, but that deprives the piece of much of its frisson and leaves it with a somewhat dated feel.
The play with mistaken identities -- the confusion of the characters about: who is the nut ? -- is quite well handled by Stoppard, but it is still a device that is far too familiar.
Enjoyable, clever, and no doubt impressive in its full orchestral setting, Every Good Boy Deserves Favour is still a relatively small and by now somewhat dated piece.